diff --git a/src/crawler.ts b/src/crawler.ts
index 1b186271..30368726 100644
--- a/src/crawler.ts
+++ b/src/crawler.ts
@@ -1,10 +1,10 @@
-import {RegistrationStore} from './registration';
-import {DatasetStore, extractIris} from './dataset';
-import {dereference, fetch, HttpError, NoDatasetFoundAtUrl} from './fetch';
+import {RegistrationStore} from './registration.js';
+import {DatasetStore, extractIris} from './dataset.js';
+import {dereference, fetch, HttpError, NoDatasetFoundAtUrl} from './fetch.js';
 import DatasetExt from 'rdf-ext/lib/Dataset';
 import Pino from 'pino';
-import {Validator} from './validator';
-import {crawlCounter} from './instrumentation';
+import {Validator} from './validator.js';
+import {crawlCounter} from './instrumentation.js';
 
 export class Crawler {
   constructor(
diff --git a/src/dataset.ts b/src/dataset.ts
index d430430f..dd747475 100644
--- a/src/dataset.ts
+++ b/src/dataset.ts
@@ -2,11 +2,11 @@ import DatasetExt from 'rdf-ext/lib/Dataset';
 import factory from 'rdf-ext';
 import rdf from 'rdf-ext';
 import {URL} from 'url';
-import {datasetType} from './query';
+import {datasetType} from './query.js';
 import {Readable, Transform} from 'stream';
 import {StreamParser} from 'n3';
 import {JsonLdParser} from 'jsonld-streaming-parser';
-import {StandardizeSchemaOrgPrefixToHttps} from './transform';
+import {StandardizeSchemaOrgPrefixToHttps} from './transform.js';
 
 export interface DatasetStore {
   /**
diff --git a/src/fetch.ts b/src/fetch.ts
index 4da7f793..f07c3bef 100644
--- a/src/fetch.ts
+++ b/src/fetch.ts
@@ -3,11 +3,11 @@ import factory from 'rdf-ext';
 import DatasetExt from 'rdf-ext/lib/Dataset';
 import {URL} from 'url';
 import {Store} from 'n3';
-import {bindingsToQuads, selectQuery, sparqlLimit} from './query';
+import {bindingsToQuads, selectQuery, sparqlLimit} from './query.js';
 import {pipeline} from 'stream';
-import {StandardizeSchemaOrgPrefixToHttps} from './transform';
+import {StandardizeSchemaOrgPrefixToHttps} from './transform.js';
 import Pino from 'pino';
-import {rdfDereferencer} from './rdf';
+import {rdfDereferencer} from './rdf.js';
 /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
 import _ from './comunica-config.json';
 
diff --git a/src/graphdb.ts b/src/graphdb.ts
index 465d47e5..d697afc5 100644
--- a/src/graphdb.ts
+++ b/src/graphdb.ts
@@ -9,8 +9,8 @@ import {
   AllowedRegistrationDomainStore,
   Registration,
   RegistrationStore,
-} from './registration';
-import {DatasetStore, extractIris} from './dataset';
+} from './registration.js';
+import {DatasetStore, extractIris} from './dataset.js';
 
 export type SparqlResult = {
   results: {
diff --git a/src/instrumentation.ts b/src/instrumentation.ts
index 2298ddf8..64250e64 100644
--- a/src/instrumentation.ts
+++ b/src/instrumentation.ts
@@ -6,7 +6,7 @@ import {
   PeriodicExportingMetricReader,
 } from '@opentelemetry/sdk-metrics';
 import {OTLPMetricExporter} from '@opentelemetry/exporter-metrics-otlp-proto';
-import {DatasetStore} from './dataset';
+import {DatasetStore} from './dataset.js';
 
 export function startInstrumentation(datasetStore: DatasetStore) {
   datasetsCounter.addCallback(async result =>
diff --git a/src/main.ts b/src/main.ts
index 994d6106..e6b04812 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -3,13 +3,13 @@ import {
   GraphDbClient,
   GraphDbDatasetStore,
   GraphDbRegistrationStore,
-} from './graphdb';
-import {readUrl, ShaclValidator} from './validator';
-import {Crawler} from './crawler';
+} from './graphdb.js';
+import {readUrl, ShaclValidator} from './validator.js';
+import {Crawler} from './crawler.js';
 import {scheduleJob} from 'node-schedule';
-import {server} from './server';
+import {server} from './server.js';
 import Pino from 'pino';
-import {startInstrumentation} from './instrumentation';
+import {startInstrumentation} from './instrumentation.js';
 
 const client = new GraphDbClient(
   process.env.GRAPHDB_URL || 'http://127.0.0.1:7200',
diff --git a/src/server.ts b/src/server.ts
index a6fd29b8..b3f1ddd3 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -6,26 +6,26 @@ import fastify, {
   FastifyRequest,
   FastifyServerOptions,
 } from 'fastify';
-import {Validator} from './validator';
+import {Validator} from './validator.js';
 import datasetExt from 'rdf-dataset-ext';
-import {dereference, fetch, HttpError, NoDatasetFoundAtUrl} from './fetch';
+import {dereference, fetch, HttpError, NoDatasetFoundAtUrl} from './fetch.js';
 import DatasetExt from 'rdf-ext/lib/Dataset';
 import {URL} from 'url';
 import {
   AllowedRegistrationDomainStore,
   Registration,
   RegistrationStore,
-} from './registration';
-import {DatasetStore, extractIris, load} from './dataset';
+} from './registration.js';
+import {DatasetStore, extractIris, load} from './dataset.js';
 import {IncomingMessage, Server} from 'http';
 import * as psl from 'psl';
-import {rdfSerializer} from './rdf';
+import {rdfSerializer} from './rdf.js';
 import fastifySwagger from '@fastify/swagger';
 import fastifyCors from '@fastify/cors';
 import {DatasetCore} from 'rdf-js';
 import acceptsSerializer from '@fastify/accepts-serializer';
 import fastifySwaggerUi from '@fastify/swagger-ui';
-import {registrationsCounter, validationsCounter} from './instrumentation';
+import {registrationsCounter, validationsCounter} from './instrumentation.js';
 
 const serializer =
   (contentType: string) =>
diff --git a/src/validator.ts b/src/validator.ts
index 364380db..9d138ad8 100644
--- a/src/validator.ts
+++ b/src/validator.ts
@@ -3,7 +3,7 @@ import factory from 'rdf-ext';
 import SHACLValidator from 'rdf-validate-shacl';
 import DatasetExt from 'rdf-ext/lib/Dataset';
 import ValidationReport from 'rdf-validate-shacl/src/validation-report';
-import {rdfDereferencer} from './rdf';
+import {rdfDereferencer} from './rdf.js';
 
 export interface Validator {
   /**